docs: def make_cache_key(custom_kbs_files=None):
      """Create cache key for kbs caches instances
  
      This function generates a unique key for a given set of arguments.
  
      The files dictionary is transformed like this:
      {'journal': '/var/journal.kb', 'books': '/var/books.kb'}
      to
      "journal=/var/journal.kb;books=/var/books.kb"
  
      Then _inspire is appended if we are an INSPIRE site.
      """
      if custom_kbs_files:
          serialized_args = ('%s=%s' % v for v in iteritems(custom_kbs_files))
          serialized_args = ';'.join(serialized_args)
      else:
          serialized_args = "default"
      cache_key = md5(serialized_args).digest()
      return cache_key